Distribution transformers used in electrical power distribution systems are known to fail from time to time. The dielectric failure of distribution transformer insulation causes a fault current to flow in the affected winding sections. This results in the fusing and/or welding of winding conductors in the immediate vicinity of the fault, so that the winding becomes locally either open-circuited or short-circuited. The severity of either condition will vary greatly, depending on the extent of damage to the winding. Even where winding damage is minor, the cutout fuse usually interrupts the fault current and removes the transformer from the system.
When a distribution transformer is spontaneously removed from service by a blown transformer fuse and no apparent cause can be found by line service personnel, current practise is to attempt to re-energize the transformer by replacing the fuse. In general, less than 10% of such failures are due to a faulty transformer. However, in the small but significant number of cases where the failure is due to internal damage to the transformer, re-energization causes catastrophic failure which can cause serious injuries to service personnel. Apart from being unsafe, the current practise can be costly due to repeat service calls to a malfunctioning transformer that has not been identified as the problem, and removal of properly working transformers from service where the actual problem cannot be located.
It is thus advantageous to be able to test a distribution transformer in the field prior to re-energization. It is further advantageous to be able to carry out some extent of transformer analysis without disconnecting the secondary load, to minimize disruption of electrical service to users.
The present invention provides a method and apparatus for in-situ fault analysis of electrical distribution transformers, which provides a high degree of reliability in detecting winding faults. The analyzer of the present invention provides sufficient information that usually the particular fault condition can be identified, often with the user's load still connected to the secondary terminals.
Transformer analysis according to the present invention relies on patterns of parameter variation in transformer winding ratios, winding resistance and magnetizing impedance, and comparison of these parameters to predetermined values. A fault condition is indicated by any significant deviation from the predetermined range. In a preferred embodiment of the invention this ratio analysis includes an analysis of transformer ratio balance and magnetizing impedance balance. The latter parameters significantly increase the likelihood of identifying certain types of fault conditions.
The transformer analyzer may also be used in the no load configuration to test surplus or refurbished transformers, and remove faulty units prior to disposal. In the former case, marginally faulty units will be prevented from being reintroduced into service; in the latter case, intact transformers or those with repairable fault types may be identified and salvaged.